1. Field of the Invention
This invention relates to the recovery of Group VIII noble metals, more particularly of palladium, from dilute aqueous solutions thereof.
2. Prior Art
It has been proposed by Hirschberg, in U.S. Pat. No. 3,294,483, that palladium can be recovered from dilute acidic palladium-containing solutions by treating the solutions with gaseous hydrogen in the presence of palladinated or platinized carriers, such as carbon black, so as to precipitate metallic palladium on the carrier. It is further proposed that the metallic palladium thus precipitated can be recovered in the form of a concentrated solution by treating the palladium-coated carrier with an oxidizing acid or with a combination of a non-oxidizing acid and an oxidizing agent, for example, hydrogen peroxide, oxygen or air.
It has been proposed by Keith et al in U.S. Pat. No. 3,138,560, to produce palladium on carbon catalysts, under neutral, but preferably alkaline, conditions, using relatively high concentrations of palladium salt in the presence of an oxidizing agent, such as hydrogen peroxide. The carbon black employed is said to have pH greater than about 4.5, preferably greater than about 7, in an aqueous dispersion.
It has been proposed by Panesko, in U.S. Pat. No. 3,694,370, that palladium can be recovered from an alkaline medium containing a mixture of palladium, rhodium and technetium by treatment with activated charcoal, which selectively adsorbs palladium before either rhodium or technetium.
A technique for the separation of technetium, palladium, rhodium and ruthenium proposed by Moore, in U.S. Pat. No. 3,848,048, requires the use of a sequence of carbon beds, on the first of which is adsorbed a high level of a chelating agent specific for palladium, on the second of which is adsorbed a chelating agent specific for technetium and on the third of which is adsorbed a chelating agent specific for rhodium and ruthenium.
Leopard, in U.S. Pat. No. 3,469,971, has proposed recovery of a Pt-group metal from a deactivated catalyst, having a refractory inorganic oxide support, by heating the acid-treated residue of the catalyst with carbon in an oxygen-free atmosphere, heating the resulting material in a medium containing free oxygen, and dissolving iron impurities with a strong mineral acid to produce the Pt-group metal.
Carlin et al, U.S. Pat. No. 3,922,231, is representative of references disclosing the use of ion-exchange resins for the recovery of palladium, rhodium and technetium from alkaline waste solutions.
It will be apparent that the prior art broadly discloses the use of carbon or treated carbon for adsorption of one or more of the Group VIII noble metals from their solutions, but that the processes disclosed are either complex, time-consuming or require the use of expensive reagents during the adsorption step. None of the prior art successfully addresses the problem of recovering Group VIII noble metals from very dilute solutions.
It is an object of this invention to provide a simple process for the efficient removal of low concentrations of palladium or other Group VIII noble metals from dilute acidic or neutral solutions.